I went round in circles for a few hrs trying to get this working. The problem was that the advice I could find was either incomplete or pertained to FreeBSD 8.x or below. It seems methods have changed a bit for ipv6 under FreeBSD 9.x
Here's what I did after creating the tunnel at HE:
edit rc.conf with the following. Substitute your nic for re0.
...then reboot, then as root run the usual commands you'd use to get gif0 up.